Mulitplexed optical coherence tomography

ABSTRACT

A system and method for surface inspection of an object using multiplexed optical coherence tomography (OCT) is provided. The method includes moving the object relative to two or more scanner heads along a direction of travel; alternatingly directing a sample beam to each of the two or more scanner heads; and when the sample beam is directed at each respective scanner head, scanning comprising: steering the sample beam from the respective scanner head to an unscanned region on the surface of the object; and performing an A-scan of the object.

TECHNICAL FIELD

The following relates generally to imaging and more specifically to asystem and method for multiplexed optical coherence tomography.

BACKGROUND

In many applications, imaging can be used to garner information about aparticular object; particularly aspects about its surface or subsurface.One such imaging technique is tomography. A device practicing tomographyimages an object by sections or sectioning, through the use of apenetrating wave. Conventionally, tomography can be used for variousapplications; for example, radiology, biology, materials science,manufacturing, quality assurance, quality control, or the like. Sometypes of tomography include, for example, optical coherence tomography,x-ray tomography, positron emission tomography, optical projectiontomography, or the like.

Conventionally, the above types of tomography, and especially opticalcoherence tomography, produce detailed imaging of an object; however,inaccuracies and problems can arise with respect to properly imaging theobject.

SUMMARY

In an aspect, there is provided a method of surface inspection of anobject using multiplexed optical coherence tomography (OCT), the methodcomprising: moving the object relative to two or more scanner headsalong a direction of travel; alternatingly directing a sample beam toeach of the two or more scanner heads; and when the sample beam isdirected at each respective scanner head, scanning comprising: steeringthe sample beam from the respective scanner head to an unscanned regionon the surface of the object; and performing an A-scan of the object.

In a particular case, the sample beam is steered at a first unscannedregion or an unscanned region adjacent to a most-recently scanned regionalong a direction opposite the direction of travel.

In another case, when the sample beam is directed at each respectivescanner head, the scanning further comprising: steering the sample beamfrom the respective scanner head to an adjacent unscanned regionadjacent to the most-recently scanned region along a direction oppositethe direction of travel; and performing an A-scan of the object.

In yet another case, when the sample beam is directed at each respectivescanner head, the scanning further comprising: determining whether thesurface of the unscanned region is in focus; and where the surface ofthe unscanned region is not in focus, adjusting an optical path in theOCT such that the surface of the unscanned region is in focus.

In yet another case, determining whether the surface of the unscannedregion is in focus comprises measuring a distance between the respectivescanner head and the surface of the object at the unscanned region todetermine whether the surface is within a focal length of the OCT withthe respective scanner head.

In yet another case, the method further comprising retrieving a surfacegeometry of the object, wherein determining whether the surface of theunscanned region is in focus comprises determining whether the surfaceis within a focal length of the OCT with the respective scanner headusing the surface geometry of the object to determine a distance betweenthe respective scanner head and the surface.

In yet another case, when the sample beam is directed at each respectivescanner head, the scanning further comprising: determining whether adifference between a reference path of the OCT and a sample path of theOCT at the unscanned region are within a coherence length; and where thedifference is not within the coherence length, adjusting an optical pathin the OCT such that the difference is within the coherence length.

In yet another case, successively scanned regions are non-adjacent.

In another aspect, there is provided a system for surface inspection ofan object using an optical coherence tomography (OCT) system, the OCTsystem comprising an optical source to produce an optical beam, a beamsplitter to direct a first derivative of the optical beam to areflective element and a second derivative of the optical beam to theobject via two or more scanner heads and direct the optical beamsreturned from the reflective element and the object to a detector fordetection of an interference effect, the system for surface inspectioncomprising: an object translator to move the object relative to the twoor more scanner heads along a direction of travel; a multiplexing moduleto use a multiport selector to alternatingly direct a sample beam toeach of the two or more scanner heads, the sample beam comprising thesecond derivative of the optical beam; and a steering module to, whenthe sample beam is directed at each respective scanner head, use a beamsteering device at the respective scanner head to steer the sample beamfrom the respective scanner head to an unscanned region on the surfaceof the object, the OCT system scanning at the unscanned region byperforming and outputting an A-scan of the object.

In a particular case, the steering module steers the sample beam at afirst unscanned region or an unscanned region adjacent to amost-recently scanned region along a direction opposite the direction oftravel.

In another case, when the sample beam is directed at each respectivescanner head, the steering module steers the sample beam from therespective scanner head to an adjacent unscanned region adjacent to themost-recently scanned region along a direction opposite the direction oftravel, the OCT system scanning at the adjacent unscanned region byperforming and outputting an A-scan of the object.

In yet another case, the system further comprising a distance module to,when the sample beam is directed at each respective scanner head,determine whether the surface of the unscanned region is in focus and,where the surface of the unscanned region is not in focus, adjust anoptical path in the OCT with an optical delay line or a focal-lengthadjusting mechanism such that the surface of the unscanned region is infocus.

In yet another case, each respective scanner head has a separate opticaldelay line or focal-length adjusting mechanism.

In yet another case, the system further comprising a distancedetermination module to measure a distance between the respectivescanner head and the surface of the object at the unscanned region suchthat the distance module can determine whether the surface is within afocal length of the OCT with the respective scanner head.

In yet another case, the system further comprising a distancedetermination module to retrieve a surface geometry of the object suchthat the distance module can determine whether the surface is within afocal length of the OCT with the respective scanner head using thesurface geometry of the object to determine a distance between therespective scanner head and the surface.

In yet another case, the system further comprising a distance module to,when the sample beam is directed at each respective scanner head,determine whether a difference between a reference path of the OCT and asample path of the OCT at the unscanned region are within a coherencelength, and where the difference is not within the coherence length,adjust an optical path in the OCT with an optical delay line or afocal-length adjusting mechanism such that the difference is within thecoherence length.

In yet another case, each respective scanner head has a separate opticaldelay line or focal-length adjusting mechanism.

In yet another case, successively scanned regions are non-adjacent.

In yet another case, the multiport selector comprises a resonatormirror.

In yet another case, the multiport selector comprises a rotatablepolygon mirror.

These and other aspects are contemplated and described herein. It willbe appreciated that the foregoing summary sets out representativeaspects of systems and methods to assist skilled readers inunderstanding the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention will become more apparent in the followingdetailed description in which reference is made to the appended drawingswherein:

FIG. 1 is schematic diagram of a multiplexed optical coherencetomography (OCT) system, according to an embodiment;

FIG. 2 is a schematic diagram for a computing module, according to thesystem of FIG. 1;

FIG. 3 is a flowchart for a method for surface inspection of an objectusing multiplexed optical coherence tomography (OCT), according to anembodiment;

FIG. 4 is a diagrammatic side view of a scanner head and object,according to an embodiment of the system of FIG. 1;

FIG. 5 is a flowchart for a method for surface inspection of an objectusing multiplexed optical coherence tomography (OCT), according toanother embodiment;

FIG. 6 is a diagrammatic side view of a scanner head and object,according to another embodiment of the system of FIG. 1;

FIG. 7A is an exemplary B-scan in which a defect was detected in a paintlayer of a vehicle part;

FIG. 7B is a plot of a score produced by for the exemplary B-scan ofFIG. 7A;

FIG. 8A is an exemplary B-scan, in which the system determined there areno features present;

FIG. 8B is an exemplary B-scan, in which the system determined there arefeatures present.

FIG. 9 is an exemplary image captured to form a top-level surface viewof an object;

FIG. 10A is an exemplary B-scan of an object without problematic defectsor features;

FIG. 10B is an exemplary A-scan of an object;

FIG. 11 is an exemplary B-scan of an object with problematic defects orfeatures;

FIG. 12 is an exemplary B-scan of an object for determining whetherthere are defects;

FIG. 13 is an exemplary B-scan of an object, showing a defect; and

FIGS. 14A and 14B illustrate, at respectively different angles ofperspective, an exemplary C-scan.

DETAILED DESCRIPTION

Embodiments will now be described with reference to the figures. Forsimplicity and clarity of illustration, where considered appropriate,reference numerals may be repeated among the Figures to indicatecorresponding or analogous elements. In addition, numerous specificdetails are set forth in order to provide a thorough understanding ofthe embodiments described herein. However, it will be understood bythose of ordinary skill in the art that the embodiments described hereinmay be practiced without these specific details. In other instances,well-known methods, procedures and components have not been described indetail so as not to obscure the embodiments described herein. Also, thedescription is not to be considered as limiting the scope of theembodiments described herein.

Various terms used throughout the present description may be read andunderstood as follows, unless the context indicates otherwise: “or” asused throughout is inclusive, as though written “and/or”; singulararticles and pronouns as used throughout include their plural forms, andvice versa; similarly, gendered pronouns include their counterpartpronouns so that pronouns should not be understood as limiting anythingdescribed herein to use, implementation, performance, etc. by a singlegender; “exemplary” should be understood as “illustrative” or“exemplifying” and not necessarily as “preferred” over otherembodiments. Further definitions for terms may be set out herein; thesemay apply to prior and subsequent instances of those terms, as will beunderstood from a reading of the present description.

Any module, unit, component, server, computer, terminal, engine ordevice exemplified herein that executes instructions may include orotherwise have access to computer readable media such as storage media,computer storage media, or data storage devices (removable and/ornon-removable) such as, for example, magnetic disks, optical disks, ortape. Computer storage media may include volatile and non-volatile,removable and non-removable media implemented in any method ortechnology for storage of information, such as computer readableinstructions, data structures, program modules, or other data. Examplesof computer storage media include RAM, ROM, EEPROM, flash memory orother memory technology, CD-ROM, digital versatile disks (DVD) or otheroptical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, or any other medium which canbe used to store the desired information and which can be accessed by anapplication, module, or both. Any such computer storage media may bepart of the device or accessible or connectable thereto. Further, unlessthe context clearly indicates otherwise, any processor or controller setout herein may be implemented as a singular processor or as a pluralityof processors. The plurality of processors may be arrayed ordistributed, and any processing function referred to herein may becarried out by one or by a plurality of processors, even though a singleprocessor may be exemplified. Any method, application or module hereindescribed may be implemented using computer readable/executableinstructions that may be stored or otherwise held by such computerreadable media and executed by the one or more processors.

The following relates generally to imaging and more specifically to asystem and method for multiplexed optical coherence tomography.

Optical coherence tomography (OCT), and particularly non-destructiveOCT, is a technique for imaging in two or three-dimensions. OCT canprovide a relatively high resolution, potentially up to few micrometers,and can have relatively deep penetration, potentially up to a fewmillimeters, in a scattering media.

OCT techniques can use back-scattered light from an object to generateinformation about that object; for example, generating athree-dimensional representation of that object when different regionsof the object are imaged.

FIG. 1 illustrates a schematic diagram of a multiplexed OCT system 100,according to an embodiment. The OCT system 100 includes an opticalsource (or photonic emitter) 102, a reflective element 104 (for example,a mirror), a beam splitter 110, and a detector (for example, aphotodetector) 106. For exemplary purposes, the diagram shows an object108 with three layers of depth. The optical source 102 produces anoriginating optical beam (or path) 112 that is directed towards the beamsplitter 110. The beam splitter 110 divides the originating beam 112 anddirects one derivative beam, referred to as the reference beam (or path)114, towards the reflective element 104 and another derivative beam,referred to herein as the sample beam (or path) 120, towards the objectto be scanned 108. Both derivative beams 114, 120 are directed back tothe beam splitter 110, and then directed as a resultant beam 118 to thedetector 106.

In the present embodiment, the system 100 includes an optical delay line115 in the reference path 114. The optical delay line 115 can be used toadjust the length of the optical path along the reference path.Particularly, the optical delay line 115 can be used to match thedistance of the sample path 120 to enable low-coherence interferometry.The optical delay line 115 can include, for example, a collimation lensdirected at a mirror 104 on a linear actuator; whereby linear actuationof the mirror 104 in the axial direction adjusts the length of thereference path 114.

In the present embodiment, the system 100 includes two or more scannerheads 121 in the sample path 120. Each of the scanner heads 121 directsthe sample beam 120 onto the object 108 when that respective scannerhead 121 receives the sample beam 120. The system 100 also includes amultiport selector 130 in the sample path 120 in between the beamsplitter 110 and the two or more scanner heads 121. As described herein,the multiport selector 130 determines which of the two or more scannerheads 121 receives the sample optical beam 120 by selecting the path ofthe sample beam 120. The multiport selector 130 can direct the samplebeam to each of the two or more scanner heads 121 over time.

In some cases, each of the scanner heads 121 can include a beam steeringdevice 125 to direct light to the object 108. The beam steering device125 may be, for example, a mirror galvanometer in one or two dimensions,a single axis scanner, a microelectromechanical system (MEMs)-basedscanning mechanism, a rotating scanner, or other suitable mechanism forbeam steering. The beam steering device 125 may be controlledelectromechanically.

In some embodiments, the scanner head 121 can also include afocal-length adjusting mechanism 123 to adjust the focal length of thesample beam 120. In some cases, the focal-length adjusting mechanism 123can be, for example, a focus-tuneable lens. In a particular example, thefocus-tuneable lens can be a liquid lens. With liquid lenses, each lensis filled with an optical liquid. When a user or system applies avoltage, the change in voltage alters the pressure profile of theliquid, resulting in a change in radius of curvature to each lens. Thischange in radius causes the lens to change the effective focal length ofthe sample beam 120, for example, in the range of +15 to +120 mm usingan aperture of 10 mm. In further cases, the focal-length adjustingmechanism 123 can be two or more lenses that are mechanically translatedrelative to each other, thereby changing the effective focal length ofthe sample beam 120.

In some cases, the OCT system 100 can include a distance determinationmodule 127 for determining the distance between the scanner head and theobject 108. In an example, the distance determination module 127 can bean infrared line scanner, laser rangefinder, 3D laser scanner, radarbased rangefinder, or the like. In some cases, the distancedetermination module 127 may be associated with, or separate from, thescanner head 121.

In some cases, the system 100 can include an amplification mechanism;for example, a doped fiber amplifier, a semiconductor amplifier, a Ramanamplifier, a parametric amplifier, or the like. The amplificationmechanism can be used to amplify the signal of the optical source 102and/or to increase quantity of photons backscattered off the surfaceunder inspection and collected on the detector 106. By using theamplification mechanism, sensitivity of the system 100 may be increased.

In some cases, the system 100 can include an object translator 109 tomove the object relative to the fixed sample beam 120 and/or the fixedscanner head 121. The object translator 109 can be, for example, aconveyor system, a robotic system, or the like. The illustration of FIG.1 is only diagrammatic as the optical paths can be comprised of opticalcables, and as such, the system components can have any number ofphysical placements and arrangements.

The optical source 102 can be any light source suitable for use with aninterferometric imaging modality; for example, a laser or light emittingdiode (LED). Particularly, in some implementations, the optical source102 can be a tunable laser the wavelength of which can be altered (i.e.swept) in a controlled manner; for example, to sweep a wide wavelengthrange (e.g. 110 nm) at high speed (e.g. 20 KHz). In a particularexample, the tunable laser can have a centre wavelength of 1310 nm,wherein the wavelength of the emitted light is continuously scanned overa 110 nm range, with a scan rate of 20 kHz and a coherence length ofover 10 mm. In a further embodiment, the optical source 102 may be a lowcoherence light source such as white light or an LED. As an example,using a low coherence light source can facilitate extraction of spectralinformation from the imaging data by distributing different opticalfrequencies onto a detector array (e.g. line array CCD) via a dispersiveelement, such as a prism, grating, or other suitable device. This canoccur in a single exposure as information of the full depth scan can beacquired.

In some cases, the optical source 102 can include a collimator 103 fornarrowing the originating beam 112. In further cases, further optics maybe included in various stages of the system 100 to control or change theoptical beams. Optics may include lenses or other optical devicessuitable to control, guide, navigate, position, or the like, the lightbeam in a desired manner; as an example, an F-theta or telecentric lensmay be included. Where an F-theta or telecentric lens is used, theplanification means that the focal-length adjusting mechanism 123 onlyhas to compensate in the axial direction, along the z-axis of theoptical beam.

In further cases, software techniques may be employed for correcting oraffecting optical errors or signals.

The detector 106 can be any suitable photodetector. In a particularcase, the detector 106 can be a balanced photodetector, which can havean increased signal to noise ratio. In further cases, the detector 106may be a photoelectric-type photodetector, such as a charge-coupleddevice (CCD) or complementary metal-oxide semiconductor (CMOS). Thedetector 106 may operate by photoemission, photovoltaic, thermal,photochemical, or polarization mechanism, or other mechanism throughwhich electromagnetic energy can be converted into an electrical signal.Upon receiving the resultant beam 118, the detector 106 can convert theradiance/intensity of the resultant beam 118 into an electrical signal.In some cases, the electrical signal may then be converted to a digitalsignal, and modified by signal conditioning techniques such as filteringand amplification. In some cases, the interference pattern correspondingto the backscattered light can be converted into a signal by thedetector 106 via, for example, a high-speed digitizer.

The multiport selector 130 can be any suitable selective beam steeringdevice. As an example, the multiport selector 130 can be a resonatormirror that selects an output line for the beam based on changes inresonation. In another example, the multiport selector 130 can be arotatable polygon mirror that directs the sample beam to differentoutput lines (connected to different scanner heads 121) as the polygonmirror rotates. The steering module 290 can select which scanner head121 is to receive the sample beam (as described below) based on changesto the resonance of the resonator mirror or controlling of the rotationof the polygon mirror. In this way, resonance changes or rotation speedcan be synchronized with the time between the performance of A-scans bythe system 100 (as described below).

The OCT system 100 also includes a computing module 200. The computingmodule 200 may be locally communicatively linked or remotelycommunicatively linked, for example via a network 150, to one or moreother elements of the system 100; for example, to the optical source102, the detector 106, the object translator 109, each of the scannerheads 121, the focal-length adjusting mechanism 123, the beam steeringdevice 125, the distance determination module 127, the optical delayline 115, and the multiport selector 130. The computing module 200 maybe used for processing and analysis of imaging data provided by the OCTsystem 100. In some cases, the computing module 200 may operate as acontrol system or controller, and in other cases, may be connected to aseparate control system or controller. Further, the computing module 200may host a user-accessible platform for invoking services, such asreporting and analysis services, and for providing computationalresources to effect machine learning techniques on the imaging data.

In an embodiment, as shown in FIG. 2, the computing module 200 caninclude a number of physical and logical components, including a centralprocessing unit (“CPU”) 260, random access memory (“RAM”) 264, an inputinterface 268, an output interface 272, a network interface 276,non-volatile storage 280, and a local bus 284 enabling CPU 260 tocommunicate with the other components. CPU 260 can include one or moreprocessors. RAM 264 provides relatively responsive volatile storage toCPU 260. The input interface 268 enables an administrator to provideinput via, for example, a keyboard and mouse. The output interface 272outputs information to output devices, for example, a display orspeakers. The network interface 276 permits communication with othersystems or computing devices. Non-volatile storage 280 stores theoperating system and programs, including computer-executableinstructions for implementing the OCT system 100 or analyzing data fromthe OCT system 100, as well as any derivative or related data. In somecases, this data can be stored in a database 288. During operation ofthe system 200, the operating system, the programs and the data may beretrieved from the non-volatile storage 280 and placed in RAM 264 tofacilitate execution. In an embodiment, the CPU 260 can be configured toexecute various modules, for example, a steering module 290, amultiplexing module 292, and a distance module 294.

In an embodiment, the system 100 can use machine learning (ML) totransform raw data from the A-scan, B-scan, or C-scan into a descriptor.The descriptor is information associated with a particular defect in theobject. The descriptor can then be used to determine a classifier forthe defect. As an example, the CPU 260 can do this detection andclassification with auto-encoders as part of a deep belief network.

OCT systems 100 generally use different localization techniques toobtain information in the axial direction, along the axis of theoriginating optical beam 112 (z-axis), and obtain information in thetransverse direction, along a plane perpendicular to the axis of theoriginating beam 112 (x-y axes). Information gained from the axialdirection can be determined by estimating the time delay of the opticalbeam reflected from structures or layers associated with the object 108.OCT systems 100 can indirectly measure the time delay of the opticalbeam using low-coherence interferometry.

Typically OCT systems that employ low-coherence interferometers can usean optical source 102 that produces an optical beam 112 with a broadoptical bandwidth. The originating optical beam 112 coming out of thesource 102 can be split by the beam splitter 110 into two derivativebeams (or paths). The first derivative beam 114 can be referred to asthe reference beam (or path or arm) and the second derivative beam 120can be referred to as the sample beam (or path or arm) of theinterferometer. Each derivative beam 114, 120 is reflected back andcombined at the detector 106.

The detector 106 can detect an interference effect (fast modulations inintensity) if the time travelled by each derivative beam in thereference arm and sample arm are approximately equal; whereby “equal”generally means a difference of less than a ‘coherence length.’ Thus,the presence of interference serves as a relative measure of distancetravelled by light on the sample arm.

For OCT, the reference arm can be scanned in a controlled manner, andthe reference beam 114 can be recorded at the detector 106. Aninterference pattern can be detected when the mirror 104 is nearlyequidistant to one of the reflecting structures or layers associatedwith the object 108. The detected distance between two locations wherethe interference occurs corresponds to the optical distance between tworeflecting structures or layers of the object in the path of the beam.Advantageously, even though the optical beam can pass through differentstructures or layers in the object, OCT can be used to separate out theamount of reflections from individual structures or layers in the pathof the optical beam.

With respect to obtaining information in the transverse direction, asdescribed below, the sample beam 120 can be focused on a small area ofthe object 108, potentially on the order of a few microns, andsuccessively scanned over a region of the object 108.

In an embodiment of an OCT system, Fourier-domain can be used as apotentially efficient approach for implementation of low-coherenceinterferometry. Instead of recording intensity at different locations ofthe reference reflective element 104, intensity can be detected as afunction of wavelengths or frequencies of the optical beam 112. In thiscase, intensity modulations, as a function of frequency, are referred toas spectral interference. Whereby, a rate of variation of intensity overdifferent frequencies can be indicative of a location of the differentreflecting structures or layers associated with the object. A Fouriertransform of spectral interference information can then be used toprovide information similar to information obtained from scanning of theoptical beam.

In an embodiment of an OCT system, spectral interference can be obtainedusing either, or both, of spectral-domain techniques and swept-sourcetechniques. With the spectral-domain technique, the optical beam can besplit into different wavelengths and detected by the detector 106 usingspectrometry. In the swept-source technique, the optical beam producedby the optical source 102 can sweep through a range of opticalwavelengths, with a temporal output of the detector 106 being convertedto spectral interference.

Advantageously, employing Fourier-domain can allow for faster imagingbecause back reflections from the object can be measured simultaneously.

The resolution of the axial and transverse information can be consideredindependent. Axial resolution is generally related to the bandwidth, orthe coherence-length, of the originating beam 112. In the case of aGaussian spectrum, the axial resolution (Δz) can be: Δz=0.44*λ₀ ²/Δλ,where Δλ₀ is the central wavelength of the optical beam and Δλ is thebandwidth defined as full-width-half-maximum of the originating beam. Inother cases, for spectrum of arbitrary shape, the axial spread functioncan be estimated as required.

In some cases, the depth of the topography imaging for an OCT system istypically limited by the depth of penetration of the optical beam intothe object 108, and in some cases, by the finite number of pixels andoptical resolution of the spectrometer associated with the detector 106.Generally, total length or maximum imaging depth z_(max) is determinedby the full spectral bandwidth λ_(ful) of the spectrometer and isexpressed by z_(max)=(¼N)*(λ₀ ²/λ_(full)) where N is the total number ofpixels of the spectrometer.

With OCT systems, sensitivity is generally dependent on the distance,and thus delay, of reflection. Sensitivity is generally related to depthby: R(z)=sin(p*z)/(p*z)*exp(−z²/(w*p)). Where w depends on the opticalresolution of spectrometer associated with the detector 106. The firstterm related to the finite pixels in the spectrometer and the secondterm related to the finite optical resolution of the spectrometer.

When implementing the OCT system 100, reflected sample and referenceoptical beams that are outside of the coherence length willtheoretically not interfere. This reflectivity profile, called anA-scan, contains information about the spatial dimensions, layers andlocation of structures within the object 108 of varying axial-depths;where the ‘axial’ direction is along the axis of the optical beam path.A cross-sectional tomograph, called a B-scan, may be achieved bylaterally combining a series of adjacent A-scans along an axisorthogonal to the axial direction. A B-scan can be considered a slice ofthe volume being imaged. One can then further combine a series ofadjacent B-scans to form a volume which is called a C-scan. Once animaging volume has been so composed, a tomograph, or slice, can becomputed along any arbitrary plane in the volume.

A-scans represent an intensity profile of the object, and its values (orprofile) characterize reflectance of the way the optical beam penetratesthe surface of the object. Thus, such scans can be used to characterizethe material from the surface of the object to some depth, at anapproximately single region of the object 108. As used in the presentdisclosure, the term ‘surface’, of an object, is understood to includethe peripheral surface down to the depth of penetration of the A-scan.B-scans can be used to provide material characterization from thesurface of the object 108 to some depth, across a contour on the surfaceof the object 108.

The system 100, as described herein, can be used to detect featuresassociated with the surface and subsurface of an object; and in somecases, for later categorization of such features. In a particular case,such features are defects in the object, due to, for example, variousmanufacturing-related errors or conditions.

In the present embodiments, as illustrated in the examples of FIGS. 4and 6, the system 100 can advantageously use time-domain OCTmultiplexing to perform multiple A-scans of an object 408 as the object408 is moved in direction 404 relative to the two or more fixed scannerheads 121.

Referring now to FIG. 3, shown therein is a method 300 of surfaceinspection using the multiplexed OCT system 100, in accordance with anembodiment. As exemplified in FIG. 4, in this case the system 100includes two scanner heads 121. The method 300 may be used forinspecting the surface of an object 408 when the object 408 is movedrelative to the scanner heads 121 along a direction of movement 404. Inan exemplary case, the method 300 can be for the purposes of detectingsurface defects or irregularities.

At block 302, the multiplexing module 292 directs the multiport selector130 to direct the sample beam 120 to a first of the beam steeringdevices 125 associated with a first of the scanner heads 121.

At block 304, the steering module 290 directs the first of the beamsteering devices 125 to steer the sample beam 120 at a first unscannedregion (identified in FIG. 4 as ‘A1’) on the surface of the object 408.

At block 306, the distance module 294 adjusts the optical delay line 115or the focal-length adjusting mechanism 123, or both, to keep the firstregion of the surface in focus and keep the reference path and samplepath approximately equal in length within the tolerance of the coherencelength. Where in focus is understood to mean having the surface within apresent focal length of the system 100.

At block 308, the system 100 performs an A-scan of the object 408.

At block 310, the multiplexing module 292 directs the multiport selector130 to direct the sample beam 120 to a second of the beam steeringdevices 125 associated with a second of the scanner heads 121.

At block 312, the steering module 290 directs the second of the beamsteering devices 125 to steer the sample beam 120 at a second unscannedregion (identified in FIG. 4 as ‘A2’) on the surface of the object 408.

At block 314, the distance module 294 adjusts the optical delay line 115or the focal-length adjusting mechanism 123, or both, to keep the secondregion of the surface in focus and keep the reference path and samplepath approximately equal in length within the tolerance of the coherencelength.

At block 316, the system 100 performs an A-scan of the object 408.

At block 318, the multiplexing module 292 directs the multiport selector130 to direct the sample beam 120 to the first of the beam steeringdevices 125.

At block 320, the steering module 290 directs the first of the beamsteering devices 125 to steer the sample beam 120 at a third unscannedregion (identified in FIG. 4 as ‘A3’) on the surface of the object 408.

At block 322, the distance module 294 adjusts the optical delay line 115or the focal-length adjusting mechanism 123, or both, to keep the thirdregion of the surface in focus and keep the reference path and samplepath approximately equal in length within the tolerance of the coherencelength.

At block 324, the system 100 performs an A-scan of the object 408.

At block 326, the multiplexing module 292 directs the multiport selector130 to direct the sample beam 120 to the second of the beam steeringdevices 125.

At block 328, the steering module 290 directs the second of the beamsteering devices 125 to steer the sample beam 120 at a fourth unscannedregion (identified in FIG. 4 as ‘A4’) on the surface of the object 408.

At block 330, the distance module 294 adjusts the optical delay line 115or the focal-length adjusting mechanism 123, or both, to keep the fourthregion of the surface in focus and keep the reference path and samplepath approximately equal in length within the tolerance of the coherencelength.

At block 332, the system 100 performs an A-scan of the object 408.

The system 100 alternates using the scanner heads 121 by consecutivelyrepeating taking A-scans with the first of the scanner heads 121 andthen with the second of the scanner heads 121 at successive unscannedregions on the surface of the object 408. Whereby prior to each A-scan,the multiplexing module 292 directs the multiport selector 130 to directthe sample beam 120 to the respective one of the scanner heads 121, thesteering module 290 directs the respective one of the beam steeringdevices 125 to steer the sample beam 120 at a respective region of theobject 408, and the optical delay line 115 or the focal-length adjustingmechanism 123 are adjusted as necessary.

In some cases, the optical delay line 115 or the focal-length adjustingmechanism 123, or both, may not be required to be adjusted at all, ornot adjusted during certain iterations, when the respective region isalready in focus and the difference between the reference path 114 andsample path 120 are within the coherence length. In further cases, theobject 408 can have a curved or modulating surface which will requireassociated adjustments in the focal length in order to keep the surfacein focus.

Referring now to FIG. 5, shown therein is a method 500 of surfaceinspection using the multiplexed OCT system 100, in accordance with anembodiment. As exemplified in FIG. 6, in this case the system 100includes three scanner heads 121. The method 500 may be used forinspecting the surface of an object 408 when the object 408 is movedrelative to the scanner head 121 along a direction of movement 404. Inan exemplary case, the method 500 can be for the purposes of detectingsurface defects or irregularities.

At block 502, the multiplexing module 292 directs the multiport selector130 to direct the sample beam 120 to a first of the beam steeringdevices 125 associated with a first of the scanner heads 121.

At block 504, the steering module 290 directs the first of the beamsteering devices 125 to steer the sample beam 120 at a first unscannedregion (identified in FIG. 6 as ‘A1’) on the surface of the object 408.

At block 506, the distance module 294 adjusts the optical delay line 115or the focal-length adjusting mechanism 123, or both, to keep the firstregion of the surface in focus and keep the reference path and samplepath approximately equal in length within the tolerance of the coherencelength.

At block 508, the system 100 performs an A-scan of the object 408.

At block 510, the multiplexing module 292 directs the multiport selector130 to direct the sample beam 120 to a second of the beam steeringdevices 125 associated with a second of the scanner heads 121.

At block 512, the steering module 290 directs the second of the beamsteering devices 125 to steer the sample beam 120 at a second unscannedregion (identified in FIG. 6 as ‘A2’) on the surface of the object 408.

At block 514, the distance module 294 adjusts the optical delay line 115or the focal-length adjusting mechanism 123, or both, to keep the secondregion of the surface in focus and keep the reference path and samplepath approximately equal in length within the tolerance of the coherencelength.

At block 516, the system 100 performs an A-scan of the object 408.

At block 518, the multiplexing module 292 directs the multiport selector130 to direct the sample beam 120 to a third of the beam steeringdevices 125 associated with a third of the scanner heads 121.

At block 520, the steering module 290 directs the third of the beamsteering devices 125 to steer the sample beam 120 at a third unscannedregion (identified in FIG. 6 as ‘A3’) on the surface of the object 408.

At block 522, the distance module 294 adjusts the optical delay line 115or the focal-length adjusting mechanism 123, or both, to keep the thirdregion of the surface in focus and keep the reference path and samplepath approximately equal in length within the tolerance of the coherencelength.

At block 524, the system 100 performs an A-scan of the object 408.

The system 100 consecutively repeats taking A-scans with the first oneof the scanner heads 121, then with the second one of the scanner heads121, and then with the third one of the scanner heads 121 at successiveunscanned regions on the surface of the object 408. Whereby prior toeach A-scan, the multiplexing module 292 directs the multiport selector130 to direct the sample beam 120 to the respective one of the scannerheads 121, the steering module 290 directs the respective one of thebeam steering devices 125 to steer the sample beam 120 at a respectiveregion of the object 408, and the optical delay line 115 or thefocal-length adjusting mechanism 123 are adjusted as necessary.

In some cases, the optical delay line 115 or the focal-length adjustingmechanism 123, or both, may not be required to be adjusted at all, ornot adjusted during certain iterations, when the respective region is infocus and the difference between the reference path 114 and sample path120 are within the coherence length. In further cases, the object 408can have a curved or modulating surface which will require associatedadjustments in the focal length in order to keep the surface in focus.

In some cases of the above embodiments, prior to adjustment of theoptical delay line 115 or the focal-length adjusting mechanism 123, thedistance module 294 can use the distance determination module 127 tomeasure a working distance between the scanner head 121 and therespective region on the surface of the object 408. In further cases,the distance module 294 can determine distance by, for example,retrieving the surface geometry of the object 408 from the database 288or from the input interface 268. As an example, the surface geometry canbe derived from a CAD model of the object 408. Upon processing theobject's surface geometry, the working distance can be determined. Infurther cases, the distance module 294 can use a combination of distancemeasurement and predetermined geometry to determine the workingdistance.

In further embodiments, the system 100 can have more than three scannerheads 121 which are sequentially used to take A-scans at successiveunscanned regions of the object 308, as exemplified in the aboveembodiments.

In further embodiments, the regions of the surface can be divided to bescanned between the various scanner heads 121 in any suitable manner; asan example, the first one of the scanner heads 121 used to performA-scans on two consecutive regions of the surface, then the second oneof the scanner heads 121 used to perform A-scans on two consecutiveregions of the surface, and so on.

In further embodiments, each region can have an A-scan performed on itmore than once via the same, or different, scanner head 121.

In further embodiments, unlike the examples of FIGS. 4 and 6, theunscanned regions of the object 408 can be spaced apart from one anotherand can be scanned by the system 100 non-sequentially. In furtherembodiments, a scanner head 121 receiving the sample beam 120 can beused to rescan a region of the object 408 previously scanned.

In the above embodiments, it is generally understood that the surface ofthe object 408 being in focus includes approximately the totality of theA-scan axial depth being in focus.

In some cases, the system 100 can perform up to approximately 1.5million A-scans per second.

While in the present embodiments the object is described as ‘moving’ viathe object translator 109, it is appreciated that moving can includesuccessively moving and stopping the object 108 for scanning orcontinuously moving the object. Additionally, while in the presentembodiments, the movement is shown along a single dimensional axis, itis appreciated that the movement of the object can be along atwo-dimensional plane.

In some cases, after the A-scans, B-scans, and/or C-scans have beendetermined, the system can detect whether there are defects in theobject using image interpretation and machine learning techniques. Thedefective label indicates that an unacceptable defect has been detected,and in some cases, such defect is of a particular type. In the examplewhere the object is a vehicle part, the defect may have different shapesand dimensions. As an example, the defect may be an unwanted round seedor crater, or the like, on or under the surface of the part. As anotherexample, the defect may have an elongated shape, such as with anunwanted fiber, or the like, on or under the surface of the part. As anexample, the acceptable/defective label may be with regards to the size,area, or volume of a defect. In another example, acceptable/defectivelabel may be with regards to the presence of defect between differentlayers of films applied in an industrial process; for example, in anautomotive setting, in an electro-deposition (ED) layer, a colour layer,or a clear layer, where each layer is in the order of tens of micronsthick.

In some cases, based on analysis of the OCT images, the system 100 canprovide further information in the form of feature localization on theobject. As an example, the information may be that there is fiber defectat location x=3.4 cm, y=5.6 cm on a vehicle part. Feature localizationcan also be specified with respect to surface depth, along the z-axis.Depth localization can be particularly advantageous in certainapplications; for example, when thin films are being applied to avehicle part. In this case, for example, after a vehicle part ispainted, paint inspection may be required on various layers including anelectro-deposition layer, a colour layer, and a clear coat layer. Beingable to detect and determine the presence of a defect between any two ofthese layers is particularly advantageous because it has implications onthe amount of re-work that may be required to resolve the imperfection.It can also be advantageous for improvement to a manufacturing processby being able to determine what type of defect is located at what layer;for example, a faulty HVAC system in the manufacturing environment couldbe responsible for introducing defects between layers. In this regard,being able to localize defect origin to a portion of the manufacturingpath is an advantage to reduce future defects and rework.

The machine-learning techniques described herein may be implemented byproviding input data to a neural network, such as a feed-forward neuralnetwork, for generating at least one output. The neural network may havea plurality of processing nodes, including a multi-variable input layerhaving a plurality of input nodes, at least one hidden layer of nodes,and an output layer having at least one output node. During operation ofa neural network, each of the nodes in the hidden layer applies afunction and a weight to any input arriving at that node (from the inputlayer or from another layer of the hidden layer), and the node mayprovide an output to other nodes (of the hidden layer or to the outputlayer). The neural network may be configured to perform a regressionanalysis providing a continuous output, or a classification analysis toclassify data. The neural networks may be trained using supervised orunsupervised learning techniques. According to a supervised learningtechnique, a training dataset is provided at the input layer inconjunction with a set of known output values at the output layer; forexample, imaging data for which defect location and/or existence isknown. During a training stage, the neural network may process thetraining dataset. It is intended that the neural network learn how toprovide an output for new input data by generalizing the information itlearns in the training stage from the training data. Training may beeffected by backpropagating error to determine weights of the nodes ofthe hidden layers to minimize the error. The training dataset, and theother data described herein, can be stored in the database 288 orotherwise accessible to the computing module 200. Once trained, oroptionally during training, test data can be provided to the neuralnetwork to provide an output. The neural network may thuscross-correlate inputs provided to the input layer in order to provideat least one output at the output layer. Preferably, the output providedby the neural network in each embodiment will be close to a desiredoutput for a given input, such that the neural network satisfactorilyprocesses the input data.

In some embodiments, the machine learning techniques can employ, atleast in part, a long short-term memory (LSTM) machine learningapproach. The LSTM neural network allows for quickly and efficientlyperforming group feature selections and classifications.

In some embodiments, the detection can be by employing, at least inpart, a convolutional neural network (CNN) machine learning approach.

While certain machine-learning approaches are described, specificallyLSTM and CNN, it is appreciated that, in some cases, other suitablemachine learning approaches may be used where appropriate.

As an example, FIG. 7A illustrates a B-scan in which a defect wasdetected in a paint layer of a vehicle part. As shown, the defect iscentered at approximately 225×10⁻² mm along the fast scan axis (x-axis).Correspondingly, FIG. 7B illustrates a plot of a score produced by theCPU 260, between 0 and 1, representing a determined possibility that adefect is present in the exemplary B-scan of FIG. 7A.

As an example, FIG. 8A illustrates a B-scan in which contours areoutlined. In this case, the CPU 260 determined that there was no defectdetected on the object. FIG. 8B also illustrates a B-scan in whichcontours are outlined. In this case, the CPU 260 determined that therewas a defect detected on the object.

FIG. 9 illustrates an exemplary image captured to form a top-levelsurface view of an object.

FIG. 10A illustrates an exemplary B-scan (cross-section) of an objectwithout problematic defects or features (i.e., a ‘clean’ surface). FIG.10B illustrates an exemplary A-scan from the center of the B-scan.

FIG. 11 illustrates an exemplary B-scan (cross-section) of an objectwith a problematic defects or feature present. In this case, as shown,there was a subsurface seed detected, centered at approximately 500along the x-axis.

FIG. 12 illustrates an exemplary B-scan of a vehicle part fordetermining whether there are painting defects. In this case, there wasno defect from the B-scan. FIG. 13 illustrates an exemplary B-scan of avehicle part for determining whether there are painting defects. In thiscase, as shown, there was a defect in the paint layer detected, centeredat approximately 225 along the x-axis.

FIGS. 14A and 14B illustrate, at respectively different angles ofperspective, an exemplary C-scan of a vehicle part. In this case, a seedwas detected as a defect in the painting of a vehicle part.

In further embodiments, machine learning can also be used by the CPU 260to detect and compensate for data acquisition errors at the A-scan,B-scan and C-scan levels.

The embodiments described herein include various intended substantialadvantages. As an example, there can be substantial cost and complexitysavings by having multiple scanner heads share a reference path and anoptical source. In another example, by using multiple scanner heads, alarger region of an object can be scanned over a shorter amount of time.

Although the invention has been described with reference to certainspecific embodiments, various modifications thereof will be apparent tothose skilled in the art without departing from the spirit and scope ofthe invention as outlined in the claims appended hereto. The entiredisclosures of all references recited above are incorporated herein byreference.

1. A method of surface inspection of an object using multiplexed opticalcoherence tomography (OCT), the method comprising: moving the objectrelative to two or more scanner heads along a direction of travel;alternatingly directing a sample beam to each of the two or more scannerheads; and when the sample beam is directed at each respective scannerhead, scanning comprising: steering the sample beam from the respectivescanner head to an unscanned region on the surface of the object; andperforming an A-scan of the object.
 2. The method of claim 1, whereinthe sample beam is steered at a first unscanned region or an unscannedregion adjacent to a most-recently scanned region along a directionopposite the direction of travel.
 3. The method of claim 1, wherein whenthe sample beam is directed at each respective scanner head, thescanning further comprising: steering the sample beam from therespective scanner head to an adjacent unscanned region adjacent to themost-recently scanned region along a direction opposite the direction oftravel; and performing an A-scan of the object.
 4. The method of claim1, wherein, when the sample beam is directed at each respective scannerhead, the scanning further comprising: determining whether the surfaceof the unscanned region is in focus; and where the surface of theunscanned region is not in focus, adjusting an optical path in the OCTsuch that the surface of the unscanned region is in focus.
 5. The methodof claim 4, wherein determining whether the surface of the unscannedregion is in focus comprises measuring a distance between the respectivescanner head and the surface of the object at the unscanned region todetermine whether the surface is within a focal length of the OCT withthe respective scanner head.
 6. The method of claim 4, furthercomprising retrieving a surface geometry of the object, whereindetermining whether the surface of the unscanned region is in focuscomprises determining whether the surface is within a focal length ofthe OCT with the respective scanner head using the surface geometry ofthe object to determine a distance between the respective scanner headand the surface.
 7. The method of claim 1, wherein, when the sample beamis directed at each respective scanner head, the scanning furthercomprising: determining whether a difference between a reference path ofthe OCT and a sample path of the OCT at the unscanned region are withina coherence length; and where the difference is not within the coherencelength, adjusting an optical path in the OCT such that the difference iswithin the coherence length.
 8. The method of claim 1, whereinsuccessively scanned regions are non-adjacent.
 9. A system for surfaceinspection of an object using an optical coherence tomography (OCT)system, the OCT system comprising an optical source to produce anoptical beam, a beam splitter to direct a first derivative of theoptical beam to a reflective element and a second derivative of theoptical beam to the object via two or more scanner heads and directoptical beams returned from the reflective element and the object to adetector for detection of an interference effect, the system for surfaceinspection comprising: an object translator to move the object relativeto the two or more scanner heads along a direction of travel; amultiplexing module to use a multiport selector to alternatingly directa sample beam to each of the two or more scanner heads, the sample beamcomprising the second derivative of the optical beam; and a steeringmodule to, when the sample beam is directed at each respective scannerhead, use a beam steering device at the respective scanner head to steerthe sample beam from the respective scanner head to an unscanned regionon the surface of the object, the OCT system scanning at the unscannedregion by performing and outputting an A-scan of the object.
 10. Thesystem of claim 9, wherein the steering module steers the sample beam ata first unscanned region or an unscanned region adjacent to amost-recently scanned region along a direction opposite the direction oftravel.
 11. The system of claim 9, wherein, when the sample beam isdirected at each respective scanner head, the steering module steers thesample beam from the respective scanner head to an adjacent unscannedregion adjacent to the most-recently scanned region along a directionopposite the direction of travel, the OCT system scanning at theadjacent unscanned region by performing and outputting an A-scan of theobject.
 12. The system of claim 9, further comprising a distance moduleto, when the sample beam is directed at each respective scanner head,determine whether the surface of the unscanned region is in focus and,where the surface of the unscanned region is not in focus, adjust anoptical path in the OCT with an optical delay line or a focal-lengthadjusting mechanism such that the surface of the unscanned region is infocus.
 13. The system of claim 12, wherein each respective scanner headhas a separate optical delay line or focal-length adjusting mechanism.14. The system of claim 12, further comprising a distance determinationmodule to measure a distance between the respective scanner head and thesurface of the object at the unscanned region such that the distancemodule can determine whether the surface is within a focal length of theOCT with the respective scanner head.
 15. The system of claim 12,further comprising a distance determination module to retrieve a surfacegeometry of the object such that the distance module can determinewhether the surface is within a focal length of the OCT with therespective scanner head using the surface geometry of the object todetermine a distance between the respective scanner head and thesurface.
 16. The system of claim 9, further comprising a distance moduleto, when the sample beam is directed at each respective scanner head,determine whether a difference between a reference path of the OCT and asample path of the OCT at the unscanned region are within a coherencelength, and where the difference is not within the coherence length,adjust an optical path in the OCT with an optical delay line or afocal-length adjusting mechanism such that the difference is within thecoherence length.
 17. The system of claim 16, wherein each respectivescanner head has a separate optical delay line or focal-length adjustingmechanism.
 18. The system of claim 9, wherein successively scannedregions are non-adjacent.
 19. The system of claim 9, wherein themultiport selector comprises a resonator mirror.
 20. The system of claim9, wherein the multiport selector comprises a rotatable polygon mirror.